This invention relates to electrostatographic reproduction machines, including printers, and more particularly to such a machine having a duplex tray assembly for handling sheets of various sizes and orientations without a risk of sheet jams due to sheet skew.
In electrostatographic printing, sheet stacking with bottom feeding are known, as is disclosed for example in U.S. Pat. No.4,632,377 issued Dec. 30, 1986 to Browse. Such stacking is useful, for example, when performing duplex copying in electrostatographic printing machines. Duplex copying, that is, copying image information to both sides of a single sheet of paper, is a well known feature of copying or printing machines. Duplex copying is desirable because it reduces the amount of paper required in a copying job, as compared to simplex (single side) copying. In duplex copying, first side sheet copies are produced and stacked temporarily in a duplex tray. When a set of first side sheet copies is complete, the sheet copies are refed seriatim out of the duplex tray and returned to the imaging loop of the machine, with an odd number of inversions in the sheet path in order to each receive a second side image properly registered on a second side thereof. The resulting duplex sheet copies are subsequently moved to an output tray or finisher.
In printing machines for performing duplex copying as such, it is known to provide a paper or sheet path in addition to imaging means that are each capable of accommodating LEF (long edge first) orientation sheets, or SEF (short edge first) orientation sheets. For example, in very low cost small machines it is desirable to provide only narrow imaging means for accommodating 81/2.times.11 inch sheets that are fed in an SEF (short edge first) orientation along a sheet path. The width or cross-path dimension of the sheet path and of the imaging elements in such a machine are therefore only required to accommodate the 81/2 inch dimension of the sheet, not the 11 inch dimension thereof. Similarly, in larger machines, it may be required to provide wider means for accommodating sheets having a cross-path edge dimension of at least 11 inches. Ordinarily, this is the long edge of an 81/2.times.11 inch sheet, fed LEF (long edge first). One current trend in the design of such machines is towards increasing the speed or number of sheet copies each machine can produce per unit time. Higher speeds as such therefore are ordinarily likely to create special sheet handling problems, such as skewing of sheets received and stacked in the duplex tray.
Skewed sheets in the duplex tray are particularly undesirable because such sheets already have a registered image on a first side thereof, and must be refed to receive a second image on its second side such that the second image is registered both to the second side and to the image on the first side. A skewed sheet in the duplex tray is likely to be refed misregistered, as well as likely to cause a jam along the sheet path.